1. Field of Invention
The invention relates to the field of chain saws and particularly to a cutting chain braking system.
2. Description of Prior Art
Manually portable chain saws typically comprise an engine-carrying housing, a cutter bar projecting forwardly from the housing, and an engine-driven cutting chain entrained for endless movement around the cutter bar. The chain saw is designed to be utilized by an operator grasping the machine with both hands. One hand of the operator normally holds a gripping handle mounted on the housing, while the other hand engages a housing control handle for actuating a trigger throttle mechanism.
In some instances of cutting operation, the cutting chain may encounter resistance of a type causing the cutter bar to kick upwardly in a manner commonly referred to as "kickback".
Generally the main method for dealing with this "kickback" phenomenon has been to incorporate a chain brake which can be actuated at the onset of "kickback", stopping the chain by means of the brake before contact with the operator.
Most chain saws incorporate a centrifugal clutch between the engine and the chain drive sprocket. A typical clutch design is illustrated in U.S. Pat. No. 2,947,411, Centrifugal Clutch Actuator and Spring Retainer by R. L. Collins, herein incorporated by reference. Since the drive sprocket is usually attached to the clutch housing, the housing typically serves as the brake drum for most braking systems.
An example of such a braking system can be found in U.S. Pat. No. 3,739,475, Chain Saw Safety Method And Apparatus by H. E. Moore. Moore provides a yoke, incorporating brake shoes pivotally mounted at each end, which is rotatably mounted to the saw housing. The brake shoes have tapered surfaces adapted to engage housing mounted lugs and thus when the yoke is rotated the brake shoes are forced into contact with the periphery of the clutch housing. The yoke is coupled to a safety bar located on the front gripping handle by means of a push rod and is spring biased to a position which retracts the brake shoes. Should the saw "kickback" and the hand or arm of the operator strike the safety bar the brake shoes are driven into the clutch drum and wedged in place stopping chain rotation.
This design has several disadvantages, one of which is that once the brake shoes are engaged they may be difficult to retract because they are wedged in place. Furthermore the braking system is relatively heavy. Another example of the use of brake shoes can be found in U.S. Pat. No. 3,793,727, Chain Saw Safety Method And Apparatus by H. E. Moore.
Subsequent designs have used flexible brake bands, i.e. U.S. Pat. No. 3,776,331, Brake-Device For Power Saws by L. Gustafsson. In this design a flexible band is loosely wrapped around the clutch drum with one end of the band attached to the housing. The opposite end is attached to one end of a lever arm which is pivotally mounted to the housing in front of the gripping handle with one end of the flexible brake band attached directly thereto. Pivotally attached to the lever is a latch adapted to engage a pin mounted in the housing. A spring mounted between the lever and housing biases the latch to the latched position and also biases the lever to the braking position. Upon movement of the lever the latch is forced past the pin and the spring continues to bias the lever to the braking position. While this design is simpler than the aforementioned designs, when the brake assembly is in the non-braking position, the spring biases the latch to the latching position and the lever to the braking position. This makes the proper selection of spring strength difficult. If the spring produces too much force it will be difficult to unlatch the safety lever and if it is too weak the braking force produced by the flexible band may not be sufficient to stop the chain as rapidly as may be desired.
Therefore it is desirable to have the spring bias the safety lever (sensing arm) in the non-braking position only when it is in the non-braking position and to only bias the lever to the braking position only when it is in that position. This ensures that there is always a positive force maintaining the desired position. This was accomplished in a design disclosed in U.S. Pat. No. 3,934,345, Snap-Acting Over Center Chain Saw Safety Brake And Method Of Operation Thereof by S. Hirschkoff. In this design a safety handle is typically mounted to the saw housing and in the actuated position holds a flexible brake band into frictional contact with the clutch drum. A guide rod is both slideably and pivotally mounted to the housing at one end and at the opposite end to a curved slot or groove. The rod is spring biased toward the safety handle. The positional relationship of the slot, rod and handle pivot point are such that in the unbraked position the thrust angle of the spring is below the pivot point of the safety handle biasing the safety handle toward the unbraked position and when the safety handle is in the braked position the thrust angle of the spring is above the pivot point biasing the safety handle toward the braked position. While this design achieves the over center latch function it is heavy and somewhat expensive to manufacture.
All the designs discussed above are actuated by an arm or hand or other appendage of the operator during kickback. However, there have been designs which have attempted to sense the angular acceleration of the chain saw upon kickback and to stop the chain without the operator having to manually move the safety lever, an example of which can be found in U.S. Pat. No. 3,923,126, Band Type Brake In A Chain Saw by E. J. Bidenset. In this design an inertia sensor in the form of a guide member and a first lever connected together are rotatably mounted in the saw housing. The first lever engages a second lever also rotatably mounted in the housing and the two are spring biased toward each other. The second lever has an arm which abuts against the end of a brake band spring biased to the braking position. In operation, when kickback occurs the inertia loads on the guide member and first lever cause them to move free of the brake band end. In another embodiment the second lever is attached directly to the end of the flexible brake band and thus rotation of the second lever directly brakes the chain.
This design is complicated and expensive to manufacture and requires a lot of care to assemble. Since the guide member is also acting as a safety bar capable of actuating the brake upon contact with the operator's hand or arm it will be difficult to set the proper triggering level. This is because the inertia due to "kickback" is going to be considerably smaller than the force applied by the operator. If the force level is indeed very low the saw will be subjected to continuous actuation of the brake by the casual touching of the guide member.
Other chain saw braking systems of interest are shown in U.S. Pat. Nos. 3,937,306, Automatic Brake Actuator For A Chain Saw by Ulf Vilhein Naslund et al; 3,964,333, Safety Braking Mechanism For A Portable Chain Saw by S. Hirschkoff; and 4,121,339, Safety Brake Mechanism For Chain Saw by M. Nikolich.
Therefore it is a primary object of this invention to provide a chain brake for a chain saw that is compact, light in weight and reliable.
Another object of this invention is to provide a chain brake for a chain saw wherein the safety bar is biased between one or the other of two positions, one being the non-braking position and the other being the braking position.
A further object of this invention is to provide a chain brake for a chain saw that requires only a small movement of the safety bar to achieve full braking effect.
A still further object of this invention is to provide a chain brake for a chain saw which provides both manual and automatic chain braking capabilities.